Welding management apparatus

ABSTRACT

A welding management apparatus for use with a tube production machine. A workpiece fed from a roll of metal strip is formed in a tubular formation having side surfaces opposite to each other. An upsetting pressure is applied to joint the opposite side surfaces of the workpiece at a jointing point. A high frequency power is supplied to the workpiece to weld the opposite side surfaces at the jointing point so as to produce a metal tube member. The welding management apparatus comprises a camera positioned to have a visual field including the jointing point for producing a video signal indicative of an image including an area luminous with a pre-arc produced in the visual field. An image processor converts the video signal into a luminance distribution pattern. An inference unit is provided for inferring a defective welding condition based upon the luminance distribution pattern. The inference unit includes an alarm unit for producing an alarm when a defective welding condition is inferred. The tube production machine employs squeeze rollers which are one piece members.

This application is a divisional of application Ser. No. 08/020,373,filed Feb. 22, 1993, now U.S. Pat. No. 5,265,787.

BACKGROUND OF THE INVENTION

This invention relates to a welding management apparatus suitable foruse with a tube production machine.

Tube production machines have been employed to produce a metal tubemember by forming a workpiece fed from a roll of metal strip in atubular formation having side surfaces opposite to each other, providingan upsetting pressure to butt the opposite side surface of the workpieceat a jointing point, and supplying a high frequency power to theworkpiece to weld the opposite side surfaces at a welding point. It isthe conventional practice to adjust the intensity of the welding heatgenerated at and near the jointing point by controlling the highfrequency power to the workpiece based upon various tube producingconditions that are sensed during the operation of the tube productionmachine. However, the welding heat intensity is dependent upon a greatnumber of tube producing conditions. It is very difficult, if notimpossible, to adjust the welding heat intensity based upon all of thetube producing conditions.

SUMMARY OF THE INVENTION

It is a main object of the invention to provide a welding managementapparatus which can achieve excellent welding heat control with ease.

Another object of the invention is to provide an improved tubeproduction machine which can produce tubes having a diameter of 8 mm orless.

There is provided, in accordance with the invention, a weldingmanagement apparatus for use with a tube production machine includingfirst means for forming a workpiece fed from a roll of metal strip in atubular formation having side surfaces opposite to each other, secondmeans for providing an upsetting pressure to joint the opposite sidesurfaces of the workpiece at a jointing point, and third means forsupplying a high frequency power to the workpiece to weld the oppositeside surfaces at the jointing point so as to produce a metal tubemember. The welding management apparatus comprises a camera positionedto have a visual field including the jointing point for producing avideo signal indicative of an image including an area luminous with apre-arc produced in the visual field, an image processor for convertingthe video signal into a luminance distribution pattern, and an inferenceunit for inferring a defective welding condition based upon theluminance distribution pattern. The inference unit includes means forproducing an alarm when a defective welding condition is inferred.

In another aspect of the invention, there is provided a weldingmanagement apparatus for use with a tube production machine includingfirst means for forming a workpiece fed from a roll of metal strip in atubular formation having side surfaces opposite to each other, secondmeans for providing an upsetting pressure to joint the opposite sidesurfaces of the workpiece at a jointing point, and third means forsupplying a high frequency power to the workpiece to weld the oppositeside surfaces at the jointing point so as to produce a metal tubemember. The welding management apparatus comprises a camera positionedto have a visual field including the jointing point for producing avideo signal indicative of an image including an area luminous with heatproduced in the visual field, a masking member placed in front of thecamera for partially masking the visual field from the camera to dividethe luminous area into first and second sections corresponding to therespective side surfaces of the workpiece, an image processor forconverting the video signal fed from the camera into a luminancedistribution pattern, and an inference unit for inferring a defectivewelding condition based upon the luminance distribution pattern.

In another aspect of the invention, there is provided a weldingmanagement apparatus for use with a tube production machine includingfirst means for forming a workpiece fed from a roll of metal strip in atubular formation having side surfaces opposite to each other, secondmeans for providing an upsetting pressure to joint the opposite sidesurfaces of the workpiece at a jointing point, and third means forsupplying a high frequency power to the workpiece to weld the oppositeside surfaces at the jointing point so as to produce a metal tubemember. The welding management apparatus comprises a camera positionedto have a visual field including the jointing point for producing avideo signal indicative of an image including an area luminous with heatproduced in the visual field, a masking member placed in front of thecamera, the masking member having a transparent window having linesextending in spaced-parallel relation to each other in a directionsubstantially normal to a direction of movement of the workpiece fordividing the visual field into a plurality of zones, an image processorfor converting the video signal fed from the camera into luminancedistribution patterns corresponding to the respective zones, and aninference unit for inferring a defective welding condition based uponthe luminance distribution patterns.

In still another aspect of the invention, there is provided a tubeproduction machine comprising means for forming a workpiece fed from aroll of metal strip in a tubular formation having side surfaces oppositeto each other, and a pair of squeeze rollers for providing an upsettingpressure to joint the opposite side surfaces of the workpiece at ajointing point. Each of the squeeze rollers is a one-piece member havinga cylindrical center portion, and upper and lower cylindrical endportions extending coaxially from the center portion. The center portionis formed in a side peripheral surface with an annular groove to definea path for the workpiece along with the annular groove of the othersqueeze roller. The tube production machine also comprises a rollerholder for bearing each of the squeeze rollers at the end portionsthereof, and means for supplying a high frequency power to the workpieceto weld the opposite side surfaces at the jointing point so as toproduce a metal tube member.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be described in greater detail by reference to thefollowing description taken in connection with the accompanyingdrawings, in which:

FIG. 1 is a schematic block diagram showing one embodiment of a weldingmanagement apparatus made in accordance with the invention;

FIG. 2 is a fragmentary perspective view showing the welding sectionincluded in a tube production machine to which the invention isapplicable;

FIG. 3 is a fragmentary plan view showing a visual field of a cameraused in the welding management apparatus of the invention;

FIG. 4 is a fragmentary plan view showing a welding heat condition whichmay appear in the visual field of the camera;

FIG. 5 is a diagram showing a luminance distribution pattern produced inthe image processor;

FIG. 6 is a diagram used in explaining the operation of the imageprocessor;

FIG. 7 is a diagram used in explaining the process for producing analarm;

FIG. 8 is a fragmentary perspective view showing another type of weldingsection included in the tube production machine to which the inventionis applicable;

FIG. 9 is a schematic block diagram showing a second embodiment of thewelding management apparatus of the invention;

FIG. 10 is a fragmentary plan view showing a visual field of a cameraused in the welding management apparatus of the invention;

FIG. 11 is a fragmentary plan view showing a welding heat conditionwhich may appear in the visual field of the camera;

FIG. 12 is a diagram showing a luminance distribution pattern producedin the image processor;

FIG. 13 is a diagram used in explaining the operation of the imageprocessor;

FIG. 14 is a diagram used in explaining the process for producing analarm;

FIG. 15 is a schematic block diagram showing a third embodiment of thewelding management apparatus of the invention;

FIG. 16 is a fragmentary plan view showing a visual field of a cameraused in the welding management apparatus of the invention;

FIG. 17 is a fragmentary plan view showing a welding heat conditionwhich may appear in the visual field of the camera;

FIG. 18 is a diagram showing a luminance distribution pattern producedin the image processor;

FIG. 19 is a diagram used in explaining the process for producing analarm;

FIG. 20 is a sectional view showing a specified form of the squeezeroller arrangement;

FIG. 21 is a plan view showing the squeeze roller arrangement of FIG.20;

FIG. 22 is an enlarged elevational view showing the squeeze rollerincluded in the squeeze roller arrangement of FIG. 20;

FIG. 23 is a fragmentary sectional view showing a modified form of thesqueeze roller arrangement;

FIG. 24 is a fragmentary plan view showing the squeeze rollers used inthe tube production machine;

FIG. 25 is a fragmentary elevational view showing the squeeze rollersused in the tube production machine;

FIG. 26 is a graph of tube diameter versus squeeze roller diameter;

FIG. 27 is a graph of tube diameter versus V throat angle;

FIG. 28 is a perspective view showing a modified form of the squeezeroller arrangement;

FIG. 29 is a plan view of the squeeze roller arrangement of FIG. 28; and

FIG. 30 is a sectional view of the squeeze roller arrangement of FIG.28.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the drawings, and in particular to FIG. 1, there isshown a schematic diagram of a welding management apparatus embodyingthe invention. Although the welding management apparatus will bedescribed in connection with a tube production machine employing a highfrequency induction welding unit, as shown in FIG. 2, it is to beunderstood that the invention is also applicable to other types of tubeproduction machine. The tube production machine, generally designated bythe numeral 10, employs forming rollers arranged in a number of stagesto form a workpiece 1 fed from a roll of metal strip in a tubularformation. A pair of squeeze rollers 13a and 13b are positioned on theopposite sides of the workpiece 1 and they provide an upsetting pressureto joint the opposite side surfaces of the workpiece 1 at a jointingpoint 1a just upstream of a point intermediate between the squeezerollers 13a and 13b, as best shown in FIG. 2. A heating coil 12 ispositioned to surround the workpiece 1 at a position upstream of thejointing point 1a. The heating coil 12 is supplied with a high frequencypower to produce a highly concentrated, rapidly alternating magneticfield so as to induce an electric potential in the workpiece 1. Thiselectric potential causes heating because of I².R losses at a weldingpoint just downstream of the jointing point 1a where the opposite sidesurfaces 1b and 1c of the workpiece 1 are welded, as best shown in FIG.2. The V-shaped gap, which is defined near the jointing point 1a by theopposite side surfaces 1b and 1c of the workpiece 1, is referred to as aV throat.

The heating coil 12 is powered from a high frequency power source 20through a power control circuit 22. The welding heat under which theworkpiece 1 is welded at the welding point 1a, this being determined bythe level of the power applied to the heating coil 12, is determinedfrom calculations performed in a control unit 30. A camera 40 ispositioned above the workpiece 1. The camera 40, which may be of thetype having an M×N array of CCD elements, is directed to have a visualfield VF including the jointing (or welding) point 1a, as shown in FIG.3. FIG. 4 shows a welding condition which may appear in the vidual fieldVF of the camera 40. In this welding condition, the opposite sidesurfaces 1b and 1c are connected by melted metal 2 at a point 2aupstream of the jointing point 1a. The melted metal 2 moves violentlybetween the points 1a and 2a and it cannot cover the whole area betweenthe points 1a and 2c to form an aperture 3 behind the melted metal 2. Anarc (referred to as pre-arc) occurs frequently at the point 2a. Thiswelding condition results from an excessive welding heat. Preferably,the camera 40 is fixed certainly to suppress the movement of the vidualfield VF within 0.1 mm at maximum. Noise will be introduced when themeasuring point moves due to vibrations in the camera 40. It is alsopreferable that the introduced noise is less than 100 μm.

The image (luminance level pattern) formed on the CCD elements isscanned in a series of raster scan lines and converted into a videosignal S1 for application to the image processing unit 50. It is to beunderstood that the image formed on the CCD elements has syntheticalinformation on which various welding conditions are reflected. The imageprocessing unit 50 receives a video signal from the camera 40 anddigitally stores the inputted image. The stored image A is representedby an M×N array of pixels. Each pixel A(x,y) is assigned a valuerepresentation of its intensity. The image processing unit 50 scans thestored image A in a series of raster scan lines to convert it into ablack/white image B represented by an array of pixels. Each pixel B(x,y)has a value of 0 or 1. B(x,y)=0 represents a white pixel, while B(x,y)=1represents a black pixel. In order to avoid the influence of the vaporand other disturbances near the jointing point 1a, the image processingunit 51 may be arranged to differentiate the signal obtained when thestored image A is scanned in a series of raster scan lines and toconvert the differentiated signal into the black/white image B. Theimage processing unit 50 counts the number of white pixels of the storedimage B and produces an electric signal S5 corresponding to the countedwhite pixel number. The electric signal S5 represents the white area ofthe stored image B and, thus, the intensity of the welding heat. Theimage processing unit 50 may be arranged to produce the electric signalS5 by counting the number of white pixels arranged to form alongitudinal center line on the stored image. Preferably, the camera 40has such a resolving power that the number of the pixels forming thewhite area (in this case pre-arc area) is 100 or more.

The image processing unit 50 may include an analog-to-digital converter(A/D) 52 which receives the video signal S1 from the camera 40 andconverts it into digital form having 128 (0 to 127) tones. That is, theluminance level (Cd/M²) taken for each of the M×N array of CCD elementsis converted into the corresponding digital signal. The digital signalS2 is applied from the A/D converter 52 to a first image memory (MEM) 54which digitally stores an image of the visual field represented by anM×N array of pixels. Each pixel is assigned a value representative ofits intensity (darkness). The stored image is scanned in a series ofraster scan lines to convert it into a luminance (gray histogram)pattern. FIG. 5 shows one example of such a luminance pattern obtainedfor the welding condition described in connection with FIG. 4. The firstluminous area surrounded by the circle P1 has the highest luminancelevel and it corresponds to the pre-arc produced at the point 2a. Thesecond luminous area surrounded by the circles P1 and P2 has a luminancelevel less than the first luminous area and it corresponds to theworkpiece portion surrounding the pre-arc. In FIG. 5, the letter Gindicates the center of gravity of the first luminous area surrounded bythe circle P1.

The image processing unit 50 also includes a second image memory (MEM)56 which stores reference luminance patterns. The digital computer (CPU)58 makes a determination as to whether or not the welding condition isappropriate by comparing the luminance pattern S3 transferred theretofrom the first image memory 54 with the reference image patterns S4successively transferred thereinto from the second image memory 56. Forthis purpose, the digital computer 58 may be arranged to compare thefirst luminous area surrounded by the circle P1 with the correspondingarea of each of the reference luminance patterns. The first luminousarea surrounded by the circle P1 corresponds to the area of the pre-arcproduced at the point 2c and it corresponds to the intensity of thewelding heat. Alternatively, the digital computer 58 may be arranged tocompare the longitudinal length of the first luminous area surrounded bythe circle P1 with the corresponding length of each of the referenceluminance patterns. The longitudinal length of the first luminous areasurrounded by the circle P1 corresponds to the length of the meltedmetal 2 and it corresponds to the intensity of the welding heat. Thedigital computer 58 produces a decision signal S5 indicative of theresult of the comparison made in the image processing unit 50. Thedecision signal S5 is fed from the image processing unit 50 to thecontrol unit 30.

The control unit 30 includes a correction factor calculating circuit(COR) 32, a signal converter circuit (CON) 34 and a signal controlcircuit (SCC) 36. The correction factor calculating circuit 32 receivesthe decision signal S5 from the digital computer 58 and a weldingcondition signal S9 and it calculates a correction factor based upon thereceived signals S5 and S9. The welding condition signal S9 may a sensorsignal indicative of at least one of the measured values of highfrequency power, high frequency impedance, welding rate, workpiecewidth, workpiece thickness, workpiece resistance, and the likeparameters having an influence on the welding quality. The correctionfactor calculating circuit 32 produces a signal S6 indicating thecalculated correction factor. The signal S6 is fed from the correctionfactor calculating circuit 32 to the signal converter circuit 34 whichconverts it into an electric signal S7. The electric signal S7 isapplied to the signal control circuit 36 which compares it with a powersetting reference signal S0 to produce a control signal S8 causing thepower control circuit 22 to control the high frequency power to theheating coil 12.

The welding management apparatus also includes an inference section 60and a monitor section 62. The monitor section 62 includes display andalarm units 64 and 66 connected to the inference section 60. Theinference section 60 receives the signals S5 and S9 and infers thewelding condition. The inference section 60 may ba arranged to utilizethe area, length, circumference, inclination angle of the first luminousarea obtained by the digital computer 58 to infer the welding condition.The inferred welding condition is presented on the display unit 64. Thealarm unit 66 operates to provide an audible indication when theinferred welding condition is defective.

The image processing unit 50 may be arranged to process the inputtedimage so as to provide an oval area Ao equivalent to the first luminousarea Ap corresponding to the pre-arc, as shown in FIG. 6. The imageprocessing unit 50 extracts at least one of characteristic featuresincluding the area of the equivalent oval area Ao, the length a of themajor axis of the equivalent oval area Ao, the length b of the minoraxis of the equivalent oval area Ao, the angle θ of inclination of theequivalent oval area Ao, the position of the gravity center of theequivalent oval area Ao, the length of the circumference of theequivalent oval area Ao, etc. The area of the equivalent oval area Aocorresponds to the magnitude of the heat (welding heat) inputted to theworkpiece 1. For example, the number of the pixels forming the firstluminous area Ao was 146 when the inputted heat is at its lower limitand was 950 when the input heat is at its upper limit. The angle θ ofinclination of the equivalent oval area Ao may be used to determinewhether the workpiece is formed well symmetrically. The position of thegravity center of the equivalent oval area Ao corresponds to the angleof the V throat defined by the opposite side surfaces 1b and 1c of theworkpiece 1. These characteristic features are presented on the displayunit 64. The inference section 60 receives the extracted characteristicfeature from the image processing unit 50 and compares the receivedcharacteristic feature with upper and lower limits. If the receivedcharacteristic feature is out of the range defined by the upper andlower limits, the inference section 60 produces a command causing thealarm unit 66 to produce an audible indication.

It is now assumed that the characteristic feature is the position of thegravity center of the equivalent oval area Ao. The inference section 60compares this characteristic feature hysteretically with a higher valueL2 (100 pixels) of the lower limit when the characteristic feature isincreasing and with a lower value L1 (80 pixels) of the lower limit whenthe characteristic feature is decreasing, as shown in FIG. 7. Similarly,the inference section 60 compares the characteristic featurehysteretically with a higher value L4 (400 pixels) when thecharacteristic feature is increasing and with a lower value L3 (350pixels) when the characteristic feature is decreasing. No alarm isproduced from the alarm unit 66 as long as the characteristic featurevaries within the acceptable range defined between the upper and lowerlimits, as indicated by the curve Co of FIG. 7. If the characteristicfeature is out of the acceptable range, as indicated by the curve C1 ofFIG. 7, the inference section 60 produces a command causing the alarmunit 66 to produce an alarm so as to indicate that the welding conditionis defective.

The inference section 60 may be arranged to infer a cause of thedefective welding condition based upon the characteristic featurechanging out of the acceptable range defined between the upper and lowerlimits. In this case, the inference section 60 indicates the inferredcause on the display unit 64.

The invention is also applicable to another type of welding machine asshown in FIG. 8. This welding machine includes a pair of contacts 14aand 14b placed in contact with the workpiece 1 on the opposite sides ofa line along which welding is required. The contacts 14a and 14b aresupplied with a high frequency power to produce an electric potential inthe workpiece 1. This electric potential causes heating because of I² Rlosses at the jointing point 1a where the opposite side surfaces 1b and1c of the workpiece 1 are welded.

Referring to FIGS. 9 and 10, there is illustrated a second embodiment ofthe welding management apparatus of the invention. The second embodimentis substantially the same as the first embodiment except for a maskingmember 42 provided in front of the camera 40 to mask the workpieceportion downstream of the welding point 1a. With the use of the maskingmember 42, the luminous area corresponding to the heated portion of theworkpiece 1 is divided into two portions 4a and 4b, as shown in FIG. 11.FIG. 12 shows one example of a luminance pattern obtained for thewelding condition shown in FIG. 11. The area P1 corresponds to the area4a of FIG. 11 and the area P2 corresponds to the area 4b of FIG. 11. Theletter G1 indicates the center of gravity of the area 4a and the letterG2 indicates the center of gravity of the area 4b.

In this embodiment, the image processing unit 50 is arranged to processthe inputted image so as to provide a trapezoid area AT1 equivalent tothe area 4a and a trapezoid area AT2 equivalent to the area 4b, as shownin FIG. 13. The image processing unit 50 extracts the areas A1 and A2 ofthe respective trapezoid areas AT1 and AT2, the lengths a1 and a2 of themajor axes of the respective trapezoid areas AT1 and AT2, the lengths b1and b2 of the minor axes of the respective trapezoid areas AT1 and AT2,the positions G1 and G2 of the gravity centers of the respectivetrapezoid areas AT1 and AT2, and the angles θ1 and θ2 of inclination ofthe respective trapezoid areas AT1 and AT2. The image processing unit 50superimposes at least one of characteristic features calculated as(A1+A2)/2, (A1-A2)/(A1+A2)^(1/2), (G1+G2)/2, |G1-G2|/(G1×G2)^(1/2), and(|θ1|-|θ2|)/(θ1.times.θ2)^(1/2), on the signal S5. The characteristicfeature (A1+A2)/2 corresponds to the magnitude of the welding heat, thethickness of the workpiece 1 and the workpiece feeding speed. Thecharacteristic feature (A1-A2)/(A1+A2)^(1/2) corresponds to the degreeto which the tubular formation of the workpiece 1 is balanced. Thecharacteristic feature (G1+G2)/2 corresponds to the upsetting pressureand the width of the workpiece 1. The characteristic feature|G1-G2|/(G1×G2)^(1/2) corresponds to the workpiece forming condition.The characteristic feature (|θ1|-|θ2|)/(θ1.times.θ2)^(1/2) correspondsto the workpiece forming stability, workpiece thickness change and thedegree to which the rollers are worn.

It is now assumed that the characteristic feature is the position of thegravity center of the equivalent area. The inference section 60 comparesthis characteristic feature hysteretically with a higher value L2 (400pixels) of an upper limit when the characteristic feature is increasingand with a lower value L1 (350 pixels) of the upper limit when thecharacteristic feature is decreasing, as shown in FIG. 14. No alarm isproduced from the alarm unit 66 as long as the characteristic featurevaries below the upper limit, as indicated by the curve Co of FIG. 14.If the characteristic feature exceeds the upper limit, as indicated bythe curve C1 of FIG. 14. The inference section 60 produces a commandcausing the alarm unit 66 to produce an alarm so as to indicate that thewelding condition is defective. The inference section 60 may be arrangedto infer a cause of the defective welding condition based upon thecharacteristic feature changing out of the acceptable range defined bythe upper limit. In this case, the inference section 60 indicates theinferred cause on the display unit 64.

Referring to FIGS. 15 and 16, there is illustrated a third embodiment ofthe welding management apparatus of the invention. The third embodimentis substantially the same as the first embodiment except for a maskingmember 44 provided in front of the camera 40. The masking member 44 hasa rectangular transparent window 44a. The transparent window 44a hasparallel lines V1-V2, V3-V4 and V5-V6 to divide vidual field (VF), thatis, the area of the transparent window 44a, into three zones V, E and F,as best shown in FIG. 17. These lines extend over the full length of thewidth of the transparent window in a direction normal to the workpiecefeeding direction A, as best shown in FIG. 17. The line V1-V2 extendsthrough the jointing point 1a of the workpiece 1. The V zone (centerzone) is defined between the line V3-V4 and the line V5-V6. The E zone,which is defined between the line V3-V4 and the window edge line E1-E2,is positioned on the downstream side of the V zone. The F zone, which isdefined between the line V5-V6 and the window edge line F1-F2, ispositioned on the upstream side of the V zone. The line C1-C2 extendsthrough the jointing point 1a in the workpiece feeding direction. One ofthe workpiece side surfaces is indicated by the line C1-B1 extending atan angle θ1 with respect to the line C1-C2. The other workpiece sidesurface is indicated by the line C1-B1 extending at an angle θ2 withrespect to the line C1-C2. With the use of the masking member 44, theluminous area corresponding to the heated portion of the workpiece 1 isdivided into three zones. FIG. 18 shows one example of luminancedistribution patterns obtained for the welding condition shown in FIG.17. The luminance distribution pattern including areas P11 and p12correspond to the F zone of FIG. 17, the luminance distribution patternincluding an area P20 corresponds to the V zone of FIG. 17, and theluminance distribution pattern including an area P30 corresponds to theE zone of FIG. 17.

In this embodiment, the image processing unit 50 is arranged to extractat least one of the following characteristic features:

(1) The area of the luminous area of the sum of the F, V and E zones,the position of the gravity center of the luminous area of the sum ofthe F, V and E zones, the length of the circumference of the luminousarea of the sum of the F, V and E zones. These characteristic featurescorrespond to the inputted (welding) heat.

(2) The angle θ1 and θ2 of the lines B1-C1 and B2-C1 with respect to theline C1-C2. To obtain the lines B1-C1 and B2-C1, the image in the areaV5-V6-F2-F1 may be differentiated. The areas A1 and A2 of the luminousareas 4a and 4b. These characteristic features correspond to the degreeto which the workpiece formation is balanced and the angle of the Vthroat.

(3) The position of the gravity center of the area V3-V4-V6-V5. Thischaracteristic feature corresponds to the magnitude of the upsettingpressure.

(4) The image in the area E1-E2-V4-V3 is differentiated to determine thepresence of slits. If slits are produced frequency, it will mean thatthe inputted (welding) heat is excessive.

It is now assumed that the characteristic feature is the position of thegravity center. The inference section 60 compares this characteristicfeature hysteretically with a higher value L2 (100 pixels) of the lowerlimit when the characteristic feature is increasing and with a lowervalue L1 (80 pixels) of the lower limit when the characteristic featureis decreasing, as shown in FIG. 19. Similarly, the inference section 60compares the characteristic feature hysteretically with a higher valueL4 (400 pixels) when the characteristic feature is increasing and with alower value L3 (350 pixels) when the characteristic feature isdecreasing. No alarm is produced from the alarm unit 66 as long as thecharacteristic feature varies within the acceptable range definedbetween the upper and lower limits, as indicated by the curve Co of FIG.19. If the characteristic feature is out of the acceptable range, asindicated by the curve C1 of FIG. 19, the inference section 60 producesa command causing the alarm unit 66 to produce an alarm so as toindicate that the welding condition is defective.

The inference section 60 may be arranged to infer a cause of thedefective welding condition based upon the characteristic featurechanging out of the acceptable range defined between the upper and lowerlimits. In this case, the inference section 60 indicates the inferredcause on the display unit 64.

Referring to Flays. 21 to 23, there is illustrated a fourth embodimentof the welding management apparatus of the invention. In thisembodiment, the tube production machine employs a pair of squeezerollers 70 each of which is a one-piece member having a cylindricalcenter portion 7Oa and upper and lower cylindrical end portions 70c and70d extending coaxially from the upper and lower surfaces of thecylindrical center portion 70a. The cylindrical end portions 70c and 70dhave a diameter somewhat shorter than the diameter D_(SQR) of thecylindrical center portion 70a. The cylindrical center portion 70a isformed in its side peripheral surface with an annular groove 70b.

The squeeze rollers 70 are juxtaposed in parallel with each other so asto applying an appropriate upsetting pressure to the curved workpiece 1passing the space defined by the annular grooves 70b of the respectivesqueeze rollers 70. The squeeze rollers 70 are supported on separateroller holders 72 each of which has upper and lower rigid arms 72aextending in spaced-parallel relation to each other. The upper and lowerrigid arms 72a are formed near their ends with coaxial holes 72b forreceipt of the upper and lower end portions 70c and 70d. The squeezeroller 70 is supported rotatably between the upper and lower arms 72awith its upper and lower end portions 70b and 70c inserted in therespective holes 72b. The roller holders 72 are bolted on respectivesliders 74 mounted for sliding movement on a roller stand (not shown). Aconventional sliding mechanism, for example, of the type including ascrew rod 76, is provided to move the sliders 74 toward and away fromeach other so as to adjust the distance between the squeeze rollers 70and thus the upsetting pressure applied to the workpiece 1. Thisarrangement can reduce the size of the squeeze rollers 70 without anyreduction in their strength. It is, therefore, possible to place thesqueeze rollers 70 closer to the induction coil 12 without interferenceof the squeeze rollers 70 with the induction coil 12 so as to improvethe welding heat efficiency. It is also possible to producesmall-diameter tubes having a diameter of 8 mm or less for which theconventional apparatus cannot be used. It is preferable to avoidinduction heating of the squeeze rollers 70 by making the squeezerollers of an insulating material such as ceramics. It is alsopreferable to provide bushes 78 for bearing the squeeze rollers 70, asshown in FIG. 23. The bushes 78 may be made of special bearing metal orthe like.

Referring to FIGS. 24 and 25, a small-diameter tube is produced whilethe curved workpiece 1 passes between the squeeze rollers 70. It is nowassumed that the squeeze rollers 70 has a maximum diameter D_(SQR) andthe small-diameter tube has a diameter Dp. If the squeeze rollerdiameter D_(SQR) increases, the distance l_(v) of the induction coil 12from the welding point 1a intermediate between the squeeze rollers 70should be increased to avoid the interference of the squeeze roller 70with the induction coil 12. The greater the distance l_(v), the greaterthe resistance of the electric path extending from the induction passage12 to the welding point 1a. As a result, the ratio of the currentflowing to the welding point 1a to the current circulating through theworkpiece 1 in the direction of the circumference of the curvedworkpiece 1 increases. Consequently, the welding heat produced at thewelding point 1a decreases for the same power applied to the inductioncoil 12. In addition, the workpiece 1 is softened by the heat conductedover the entire area of the workpiece so that the V throat cannot bemaintained at a constant angle θ.

The inventors have discovered an acceptable range of the squeeze rollerdiameter D_(SQR) from tests conducted to change the squeeze rollerdiameter D_(SQR) for workpieces having various widths ranging from 11 mmto 25 mm. The test results are illustrated in FIG. 26. As can be seenfrom the test results, it is preferable that the squeeze roller diameterD_(SQR) be in an acceptable range defined by first and second lines La1and Lb1. The first line La1 is presented as 5.5×Dp and the second lineLb1 is presented as 2.5×Dp. Assuming now that the tube diameter Dp is6.35 mm, the acceptable range of the squeeze roller diameter D_(SQR)extends from 15.9 mm to 32.9 mm. Assuming now that the tube diameter Dpis 4 mm, the acceptable range of the squeeze roller diameter D_(SQR)extends from 10.1 mm to 20.8 mm. If the squeeze roller diameter D_(SQR)is greater than the acceptable range, the distance l_(v) is too long. Ifthe squeeze roller diameter D_(SQR) is less than the acceptable range,the squeeze rollers 70 will have an insufficient strength against thereaction force produced in response to an upsetting pressure applied tothe workpiece 1.

A seam guide 80 is placed between the opposite side surfaces 1b and 1cof the workpiece 1 to increase the angle θ of the V throat so as toshorten the distance Lsg. The inventors have discovered an acceptablerange of the V throat angle θ from tests conducted to change the Vthroat angle θ for workpieces having various widths ranging from 11 mmto 25 mm. The test results are illustrated in FIG. 27. As can be seenfrom the test results, it is preferable that the V throat angle θ be inan acceptable range defined by first and second lines La2 and Lb2. Thefirst line La2 is presented as 1.5×Dp and the second line Lb2 ispresented as 0.6×Dp. Assuming now that the tube diameter Dp is 6.35 mm,the acceptable range of the V throat angle θ extends from 4.06° to8.83°. Assuming now that the tube diameter Dp is 4 mm, the acceptablerange of the V throat angle θ extends from 2.56° to 5.56°. If the Vthroat angle θ is greater than the acceptable range, the wrinkles ordeformations occur on the opposite side surfaces 1b and 1c of theworkpiece 1. If the V throat angle θ is less than the acceptable range,pre-arc occurs in the V throat of the workpiece 1 to degrade the weldingquality.

Referring to FIGS. 28 to 30, there is illustrated a fifth embodiment ofthe welding management apparatus of the invention. In this embodiment,the squeeze rollers 70 are supported in a cassette holder 80. Thecassette holder 80 includes a lower member 80a and an upper member 80bfixed on the lower member 80a to form a box-shaped holder. The upper andlower members 80a and 80b are formed near their centers with a pair ofcoaxial holes 80c for receipt of the upper and lower end portions 70cand 70d. The squeeze roller 70 is supported rotatably between the upperand lower members 80a and 80b with its upper and lower end portions 70band 70c inserted in the respective holes 80c. The cassette holder 80 isfixed on a roller stand by means of bolts 86 extending through boltsholes 82 formed in the upper and lower members 80a and 80b. Thus, thedistance between the squeeze rollers 70 are fixed at a predeterminedappropriate value. The numeral 88 designates a cutout formed in theupper member 80a intermediate between the holes 80c. The cutout 88 iseffective to monitor the welding point 1a therethrough and escapespatters therethrough.

This arrangement can reduce the size of the squeeze rollers 70 withoutany reduction in their strength. It is, therefore, possible to place thesqueeze rollers 70 closer to the induction coil 12 without interferenceof the squeeze rollers 70 with the induction coil 12 so as to improvethe welding heat efficiency. It is also possible to producesmall-diameter tubes having a diameter of 8 mm or less for which theconventional apparatus cannot be used.

While the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, it is intended to embrace all alternatives, modificationsand variations that fall within the scope of the appended claims.

What is claimed is:
 1. A tube production machine comprising:means for forming a workpiece fed from a roll of metal strip in a tubular formation having side surfaces opposite to each other; a pair of squeeze rollers for providing an upsetting pressure to joint the opposite side surfaces of the workpiece at a jointing point, each of the squeeze rollers being a one-piece member having a cylindrical center portion and upper and lower cylindrical end portions extending coaxially in opposite directions from the cylindrical center portion, the upper and lower cylindrical end portions being formed for rotation in unison with the cylindrical center portion, the cylindrical center portion being formed in a side peripheral surface with an annular groove to define a path for the workpiece along with the annular groove of the other squeeze roller; a roller holder for bearing each of the squeeze rollers at the cylindrical end portions thereof; and means for supplying a high frequency power to the workpiece to weld the opposite side surfaces at the jointing point so as to produce a metal tube member.
 2. The tube production machine as claimed in claim 1, wherein the roller holder includes first and second sections for bearing the respective squeeze rollers, the first and second sections being mounted for sliding movement toward and away from each other to position the squeeze rollers at an adjustable distance.
 3. The tube production machine as claimed in claim 1, where the roller holder is a single member for supporting the squeeze rollers at a predetermined distance.
 4. The tube production machine as claimed in claim 1, wherein the center portion has a largest diameter set within a range defined by first and second limits, the first limit being presented as 5.5×Dp, the second limit being presented as 2.5×Dp where Dp is the outer diameter of the tube to be produced.
 5. The tube production machine as claimed in claim 1, wherein the opposite side surfaces make an angle set within a range defined by first and second limits, the first limit being presented as 1.5×Dp, the second limit being presented as 0.6×Dp where Dp is the outer diameter of the tube to be produced.
 6. The tube production machine as claimed in claim 5, wherein the center portion has a largest diameter set within a range defined by third and fourth limits, the third limit being presented as 5.5×Dp, the fourth limit being presented as 2.5×Dp where Dp is the outer diameter of the tube to be produced.
 7. The tube production machine as claimed in claim 1, wherein at least one of the squeeze rollers is made of an insulating material.
 8. The tube production machine as claimed in claim 7, wherein the insulating material is a ceramic.
 9. The tube production machine as claimed in claim 1, wherein the roller holder bears each of the squeeze rollers at an outer circumferential surface of the cylindrical end portions. 